1. Field of the Invention
This invention relates to a casing for a display device such as a fluorescent display device, and more particularly to a casing for a display device of the tipless type that an evacuation tube is not used.
2. Description of the Prior Art
A fluorescent display device comprises a casing evacuated to high vacuum of about 1.times.10.sup.-5 -1.times.10.sup.-7 Torr and an electrode assembly including cathodes for emitting electrons therefrom, phosphor-deposited anodes arranged on conductors, control electrodes for carrying out the acceleration and/or control of electrons emitted from the cathodes and the like. In general, a casing for a fluorescent display device has an evacuation tube of glass inserted into an evacuation hole, through which the casing is evacuated to a high vacuum. The evacuation tube is then sealed by melting when the casing is evacuated to a high vacuum therethrough.
However, the conventional casing has a disadvantage of causing a display device to have a low space factor, because the tip tube remains projected outwardly from the casing after it is sealed. Also, the tip tube is formed of glass inferior in impact resistance, resulting in the fluorescent display device being inferior in durability.
In view of the foregoing, casings for a fluorescent display device of the tipless type have been proposed which are constructed in a manner as shown in FIGS. 1 and 2. In the casing shown in FIG. 1, a glass substrate A is formed with a through-hole B and a ceramic element D is bonded onto the inner surface of the through-hole B by means of frit glass C. The ceramic element D is formed with a through-hole E of a smaller diameter substantially concentric with the through-hole B, and a metallized layer F is deposited on the inner side surface of the through-hole E and on the surface portion of the ceramic element D adjacent to the through-hole E and opposite to the through-hole B. The hermetic sealing of the casing is carried out by heating the vicinity of the through-hole E to melt a brazing material G filled in the through-hole E. After the casing is evacuated, material G is filled in the through-hole E.
Another conventional casing for a tipless fluorescent display device has been proposed, which is constructed as shown in FIG. 2.
More particularly, a casing shown in FIG. 2 includes a glass substrate 1 constituting a part of the casing, on an upper surface of which anode conductors 2, insulating layers 3 and phosphor layers 4 are deposited in order. The anode conductors 2 each are formed of an Al film into a stripe shape or formed of a light-permeable material such as a transparent conductuve film or the like, so that the luminous display of a fluorescent display device may be observed through the substrate 1.
Stretched above the substrate 1 are filamentary cathodes 5 for emitting thermions thereform. Also, between the filamentary cathodes 5 and the substrate 1 are arranged control electrodes 6 which serve to carry out the acceleration and/or control of acceleration of the thermions to impinge them on the phosphor layers 4, to thereby cause the fluorescent display device to carry out a desired luminous display.
Reference numerals 7 and 8 respectively designate an upper plate and side plates which form the casing in cooperation with the substrate 1, and the substrate 1, upper plate 7 and side plates are integrally assembled by means of a sealer 11 formed of low-melting frit glass or the like. The upper plate 7 is formed with an evacuation hole 7a. The fluorescent display device constructed as described above is placed in an evacuating apparatus (not shown), which is then evacuated to desired vacuum to evacuate the casing through the exhaust vent 7a. The sealing of the evacuation hole 7a is carried out in a manner to heat the overall casing to subject it to a sufficient baking treatment, pressedly arrange a lid member 10 on which a sealer 9 formed of low-melting frit glass or the like was previously deposited and which was then calcined in an ambient atmosphere around the evacuation hole 7a, and heat the lid member 10 to a temperature sufficient to melt the sealer 9, to thereby seal the evacuation hole 7a.
However, the conventional casings of such construction have not been put into practice due to the following disadvantages.
One of the disadvantages in the casing shown in FIG. 1 is that the casing is hard to manufacture and complicated in structure because it is required to form the hard to work ceramic element with the through-hole E and also it is necessary to fix the ceramic element D with respect to the through-hole B of the glass substrate A from the inside of the casing in a specific atmosphere.
Another disadvantage is that a vapor of metal generated from the brazing material G and gas generated from organic flux by heating remain in the casing not only to be reactively absorbed in an oxide cathode to cause sintering, to thereby hinder the electron discharging capacity of the cathode but to cause the decrease in vacuum within the casing and the contamination of the surface of a fluorescent layer to decrease display characteristics of the fluorescent display device, because the through-hole E of the substrate A is sealedly filled with the bracing material G by melting.
The conventional casing shown in FIG. 2 accomplishes the simplication of the manufacturing process but fails to effectively control or regulate the positional relationship of the sealer 9 to the evacuation hole 7a during the sealing of the hole. Accordingly, it does not prevent the sealer 9 from flowing into the evacuation hole 7a so that decomposition gas of the sealer and the like may be generated during the sealing and then adsorbed on the cathodes 5 as indicated by arrows a in FIG. 2, as in the casing shown in FIG. 1. Thus, the casing likewise has a disadvantage of decreasing the electron emitting capability of the cathodes and causing the contamination of surface of the phosphor layers 4 to substantially deteriorate the display characteristics of the fluorescent display device.